Congenital heart disease affects approximately 1% of infants, with an incidence estimated at close to ten times that level among stillbirths. Heart defects are seen both as isolated findings and components of syndromes. Several chromosomal syndromes include heart defects as consistent parts of the phenotype. Examples of such syndromes include Down syndrome (AV canal defects), DiGeorge/velocardiofacial (conotruncal defects), Williams syndrome (supravalvular aortic stenosis, pulmonary vascular involvement), and Alagille syndrome (pulmonary artery defects). Identifying the specific genes involved in these and other complex developmental disorders have contributed to our understanding of the molecular processes involved in cardiac development. Human telomeres and subtelomeres have a unique structure consisting of multiple classes of DNA sequence repeats, as well as single copy sequences. These unique sequence regions are highly gene rich and prone to breakage and rearrangement. Consequences of this chromosome breakage have been shown to result in human disease. The development of molecular probe sets that permit the analysis of the integrity of the subtelomeric regions using fluorescence in situ hybridization (FISH) technology has recently advanced the clinical diagnosis of patients with these types of chromosomal rearrangements. Through the use of this testing we have identified over 40 cases of subtelomeric rearrangements. Twenty-five percent of these individuals have congenital heart defects. We have begun to molecularly characterize the deletion boundaries in those cases with a consistent finding of congenital heart defects (specifically chromosome 6p associated with atrial septal defects and pulmonary artery abnormalities, and 9q associated with conotruncal defects). We propose to study a cohort of patients with subtelomeric rearrangements and congenital heart defects, characterize their deletion boundaries, and identify genes within these deletions that are responsible for cardiac defects when mutated. We hypothesize that these studies will lead to the identification of genes responsible for normal cardiac development that when mutated will be responsible for both syndromic and isolated forms of congenital heart defects.